1. Field of the Invention
The present invention relates to a light emitting diode, and more particularly, to a light emitting diode having a modulation doped layer.
2. Discussion of the Background
In general, nitride-based semiconductors are widely used for ultraviolet, blue/green light emitting diodes (LEDs) or laser diodes as light sources of full-color displays, traffic lights, general illuminators and optical communication devices. Such a nitride-based light emitting device has an active region of an InGaN-based multiple quantum well structure, which is interposed between n-type and p-type nitride semiconductor layers, and emits light through recombination of electrons and holes in the active region.
FIG. 1 is a sectional view illustrating a conventional LED.
Referring to FIG. 1, the conventional LED comprises a substrate 11, a buffer layer 13, an undoped GaN layer 15, an n-type GaN contact layer 17, an active region 19, a p-type AlGaN clad layer 21, a p-type GaN contact layer 25, a transparent electrode 27, a p-electrode 29 and an n-electrode 31.
The conventional LED has an active region 19 of a multiple quantum well structure having an InGaN well layer between the n-type and p-type contact layers 17 and 25, thereby improving light emitting efficiency. Further, light having a desired wavelength can be emitted by controlling the In content of the InGaN well layer in the multiple quantum well structure.
However, a nitride-based compound semiconductor used in the conventional LED is generally grown on the heterogeneous substrate 11 such as sapphire. In this case, there may be a large lattice constant difference between sapphire and GaN crystals, and therefore, a strong tensile stress may be generated in the GaN layer grown on the sapphire substrate. The tensile stress causes high-density crystal defects, e.g., dislocations to be generated in the GaN layer, and such dislocations are transferred to the active region 19 of the multiple quantum well structure, thereby reducing the light emitting efficiency.
Further, since a lattice mismatch of 11% exists between GaN and InN, a large strain is generated on the interface between a quantum well and a quantum barrier in the InGaN-based multiple quantum well structure. The strain induces a piezoelectric field in the quantum well, thereby resulting in the reduction of internal quantum efficiency. Particularly, since the amount of In contained in a quantum well is increased in a green LED, the internal quantum efficiency is further reduced by the piezoelectric field. In addition, the strain generated in the multiple quantum well structure is influenced by an n-type nitride semiconductor layer adjacent to an active layer. As the mismatch of lattice constants between an n-type nitride semiconductor layer, e.g., an n-type contact layer, and a quantum well layer increases, the strain induced in the active region increases.
In order to reduce the strain generated in the active region, a technique is used of forming a superlattice structure in which first and second nitride semiconductor layers having different compositions are alternately laminated between an n-type GaN contact layer and an active layer. However, when a superlattice structure having nitride semiconductor layers with different compositions is formed between an n-type contact layer and an active layer, the growth conditions of the respective layers, e.g., temperatures and gas flow rates, are different. Therefore, a process of forming the superlattice structure is complicated, and process time is increased.
A conventional LED allows current to be uniformly distributed in the p-type contact layer 25 by forming the transparent electrode 27, which may be made of indium tin oxide (ITO), on the p-type contact layer 25.
However, the current distribution using the transparent electrode 27 has a limitation due to its light transmittance and resistance. That is, as the transparent electrode 27 becomes thicker, its light transmittance rapidly decreases. When the resistance of the transparent electrode 27 is excessively low, current flows into the sides of the transparent electrode 27 and then flows out through the sides of the transparent electrode 27, so that light emitting efficiency may be reduced. On the other hand, with a thin transparent electrode 27, it is difficult to uniformly distribute current on the p-type contact layer 25. Accordingly, although current distribution performance is ensured by optimizing the thickness of the transparent electrode 27, there is a limit in optimizing current distribution due to nonuniformity in thickness of the transparent electrode 27, crystal defects in the p-type contact layer 25, and the like.
Current distribution performance in an LED is closely related to an electrostatic discharge (ESD) characteristic, a turn-on voltage, and the like. When the current distribution performance is poor, the ESD characteristic is degraded, and the turn-on voltage is reduced.
A superlattice layer may be formed on the p-side region of the LED to improve the ESD characteristic. However, since a superlattice layer having nitride semiconductor layers with different compositions is typically used, manufacturing time of the LED increases.